Gas turbine engines and other turbomachines have rows of blades which rotate within a generally cylindrical case. As the blades rotate, their tips move in close proximity to the internal wall surface of the case. To maximize engine operating efficiency, the leakage of the gas or other working fluid between the blade tips and the case should be minimized. As has been known for some time, this may be achieved by blade and seal systems in which the blade tips rub against a seal attached to the interior of the case. Generally, the blade tip is made to be harder and more abrasive than the seal so that the tips cut into the seal during those portions of the engine operating cycle when they contact each other.
One type of blade tip which is particularly useful in the high temperature section of a gas turbine engine is described in U.S. Pat. No. 4,249,913 to Johnson et al, entitled "Alumina Coated Silicon Carbide Abrasive", of common ownership herewith. The contents of this patent are incorporated by reference. In the Johnson et al invention, silicon carbide abrasive particles of about 0.20-0.75 mm average diameter are coated with a metal oxide such as alumina and incorporated by powder metal techniques in nickel or cobalt base matrix alloys. A powder metal compact containing up to about 45 volume percent of these ceramic particles may be made which is then bonded to the tip of the blade. The resulting abrasive blade tip is particularly well suited for rubbing metal as well as ceramic seals.
As described in greater detail in the copending and commonly assigned application "Abrasive Surface Coating Process for Superalloys" to Eaton et al, U.S. Ser. No. 624,446, now U.S. Pat. No. 4,610,698 which is incorporated by reference, improved techniques for the fabrication of blade tips useful at high temperatures are desired. Specifically, the blade tip should be as thin as possible, and the amount of abrasive particles minimized. In order for the tip to provide the required abrasive characteristics, it is essential that the abrasive particles be securely bonded to the blade tip surface.
When an abrasive layer is provided on a superalloy turbine blade tip, its method of application must be metallurgically compatible with the superalloy substrate so that the properties of the substrate are not degraded. Such considerations place restraints on the kinds of materials and processing techniques which are useful in the fabrication of such abrasive layers.